In an image forming apparatus such as an electrophotographic copying machine, an image is formed on a recording medium (a sheet or the like) in a cycle of charging, exposure, development, transfer, and the like. For example, the surface of a photoconductive drum is uniformly charged, an electrostatic latent image is formed by illuminating the charged photoconductive drum with a laser beam, and a toner image is formed by developing the electrostatic latent image on the photoconductive drum using a developing device.
The toner image developed on the photoconductive drum is primarily transferred onto an intermediate transfer belt or the like which rotates, and then the toner image is secondarily transferred onto the sheet. The sheet onto which the toner image is transferred is heated by a fixing device, such that the toner image is fixed. The sheet having the toner image fixed is discharged by a transport roller. In addition, in the image forming apparatus, an image can be formed on a sheet supplied from a paper feeding cassette or a sheet that is manually fed.
However, in the image forming apparatus according to the related art, since the resistance of the sheet significantly varies according to grammage (g/m2) temperature, humidity, and the like, the resistance of the sheet is measured before performing the secondary transfer, and the transfer is performed by setting a suitable transfer condition (for example, a transfer bias) according to the measured resistance. In order to measure the resistance of the sheet, a bias roller and an opposing roller are disposed between a power source and the earth, and the resistance of the sheet is measured by interposing the sheet between the bias roller and the opposing roller.
However, when the humidity is high and the sheet is thin, the resistance of the sheet becomes smaller than the resistance of the opposing roller, and therefore a test current may not flow to the opposing roller but flow to the sheet side. In addition, if the sheet touches a user, the current flows to the user through the sheet, so that there is a possibility of the user receiving an electric shock.